Third order pressure gradient responsive microphone



Dec. 11, 1956 M. E. HAWLEY THIRD ORDER PRESSURE GRADIENT RESPONSIVEMICROPHONE Filed Feb. 1, 1954 INVENTOR. Mzzwflfiazwiey A TTOR NE Y THEREORDER PRESURE GRADIENT REPUNSVE MICROPHQNE Mones E. Hawley,Collingswood, N. 5., assignor, by mesne assignments, to the Uniteddtates of America as represented by the Secretary of the NavyApplication February 1, 1954, Serial No. 407,529

9 Claims. (Cl. 179-1) This invention relates to sound translatingapparatus, and, more particularly, to a third order, pressure gradientresponsive microphone.

Third order pressure gradient responsive microphones are particularlyuseful in providing microphones with highly directional characteristics,and an ability to discriminate against noise. Such microphones areuseful in numerous commercial fields, especially in televisionbroadcasting where it is desirable to pick up sound usually from asingle direction while excluding all other noises, such as those fromcamera or other sources.

A pressure gradient responsive microphone is one in which the output issubstantially proportional to a derivative order sound pressure withrespect to distance from the source. Microphones of this type areclassified according to the order of the pressure derivative. Thus, afirst order microphone has an output proportional to the firstderivative. A second order microphone has an output proportional to thesecond derivative. An nth order microphone has an output proportional tothe nth derivative. First order pressure gradient responsive microphonesmay comprise either two elements responsive to the pressure of a soundwave, or a single element responsive to the pressure gradient of a soundwave. A second order, pressure gradient, responsive microphone wouldrequire either two first order microphones or four pressure microphones.A third order microphone would require either two second ordermicrophones or four first order microphones, or eight pressuremicrophones. An nth order microphone would require 2 pressuremicrophones.

Pressure gradient responsive microphones having third order operationhave been effected with but a single pressure sensitive element. Thishas been accomplished by picking up acoustic energy from differentpoints and transmitting the energy through acoustic chambers on oppositesides of the pressure sensitive element. A number of tubes or conduitsare used to connect the ambient to the pressure sensitive elements.Considerable difficulty is encountered in constructing acousticallysymmetrical systems when a single pressure sensitive element is used.This is due to the fact that a slight variation in construction willresult in a vast difference in the acoustical impedance within thesystem.

It is an object of the present invention to provide an improvedmicrophone structure having a third order, pressure gradient responseand having improved frequency response.

It is a further object of this invention to provide an improvedmicrophone structure having a third order response which utilizes aminimum number of parts and which requires but a single pressuresensitive element.

It is a further object of this invention to provide an improvedmicrophone structure having a third order response which is relativelysimple and easy to construct.

It is a still further object of this invention to provide an improvedmicrophone structure in which the acoustical impedances within thesystem are more easily balanced.

Patented Dec. 11, 1956 In accordance with the present invention, thethird order, pressure gradient responsive microphone comprises a housingmember having a single pressure sensitive element or diaphragm. Apressure plate is disposed on each side of the diaphragm within thehousing. Each of the pressure plates has an annular groove. A pluralityof spaced apart openings or indentations connects the annular grooveswith the ambient. The sizes of the openings in the pressure plate andthe acoustical impedances of the annular grooves are controlled to bearratios to each other in a manner to provide third order operation.

Other objects and advantages of the present invention will becomeapparent and suggest themselves to those skilled in the art to which thepresent invention is directed from a reading of the followingspecification in connection with the accompanying drawing in which:

Figure 1 is a view in side elevation of a preferred embodiment of athird order pressure gradient responsive microphone in accordance withthe present invention;

Figure 2 is an exploded perspective view partly in section of themicrophone shown in Figure 1; and

Figure 3 is a schematic diagram of a simple electrical circuitequivalent to the acoustical network of the vibrating system of themicrophone shown in Figure 1.

Before referring to the accompanying drawing in greater detail, it isdesired to point out the background theory which prompted thedevelopment of the present invention. In accordance with this theory, ifit is desired to build a third order, pressure gradient responsivemicrophone, instead of using eight pressure microphones, only five arenecessary since one can utilize pressures P2, P3 and P4 twice. Thus, forthe first pressure difference one obtains (Pi-P2), for the secondpressure difference one has (P2-P3), for the third pressure differenceone has (Pa-P4) and for the fourth pressure difference (Pr-P5). Thesemay be combined and expressed as follows to obtain third orderoperation:

Expression 1 may be simplified and written as follows:

It will be noted that the third pressure P cancels out.

Therefore, it is not necessary to utilize the microphone providingpressure P at all. A third order, pressure gradient responsivemicrophone may be constructed using a single pressure-sensitive element.According to Expression 1 above, five pressure-sensitive elements mustbe considered in the case of a third order microphone. These elementsmay be denoted as the first, second, third, fourth and fifth elementsrespectively. Although it is necessary to take into account all fiveelements, Expression 2 shows that only four elements are actuallynecessary. The output of the third element is not used but is taken intoaccount in the spacing of the other four elements. In other words, athird order microphone may be provided by admitting acoustic energy to asingle vibratory element or diaphragm through four apertures (denotedherein as first, second, third and fourth elements corresponding,respectively, to the pressure at the first, second, fourth and fifthpositions in space referred to in Equation 2 above), directing theenergy to one side of the diaphragm through the first and thirdapertures and to the other side of the diaphragm through the second andfourth apertures and making the diaphragm movement twice as sensitive toa unit quantity of energy entering either the second or third aperturesas to a unit quantity of energy entering either the first or fourthapertures.

Referring particularly to Figures 1 and 2 of the drawing, there is shownan end view of a preferred embodiment of the invention. Anelectro-mechanical inverter 2, is mounted on the exterior of a casing orhousing member 4 by a suitable attaching means and is connected to adiaphragm 6 by a drive rod 8. The drive rod extends through an opening 9in a pressure plate it The casing member has four apertures l2, l4, l6and lit to permit the acoustic chambers on the two sides of thediaphragm to be connected to the ambient, as will be subsequentlydescribed.

The pressure plate It) consists of a circular disc having an annulargroove 26). A pair of slots or indentations 22 and 24 are out along theperipheral edge of the disc and serve to connect an acoustic chamberformed by the annular groove 2% within the pressure plate to the ambientwhen the pressure plate is assembled and placed against the peripheraledge of a diaphragm 6. The central portion 26 of the pressure plate isdisc shaped and adapted to be the same shape as the diaphragm. A washerSt? is adapted to provide proper spacing between the pressure plate litand the diaphragm 6 when the unit is assembled. The diaphragm 6comprises a central dished portion 32 having an upraised portion 33 andan integral, annular, peripheral portion 34. stiffening flutes or ribs36 are provided between the central portion and the periphery of thediaphragm. This type of diaphragm permits enhanced vibration to beobtained. A washer or spacing member 33 is used to engage the peripheralportion of the diaphragm and to provide proper spacing between thediaphragm and a second pressure plate 4h. The second pressure plate alsohas an annular groove 42 to provide a second acoustic chamber and a pairof slot openings or indentations 44 and as to permit the acousticchamber of the diaphragm to be connected to the ambient. The centralpart of the second pressure plate is provided with a raised portion 43of substantially the same shape as the corresponding portion of thediaphragm. A central opening St) is provided to permit an adjustablespacing member "52 to be inserted therein. A pair of set screws 54 and56 are provided to hold the spacing member in place. A holdoif screw 52;is provided to maintain the operating member at the desired distance.This permits a certain amount of balancing between the acoustic chamberson the two sides of diaphragms to be attained.

The assembled microphone unit comprising the two pressure plates withthe diaphragm and the washers or spacing members mounted therebetween isdesigned to fit into the cylindrical casing The circular ring so is designed to threadedly engage the inner portion of the easing and to exerta pressure against the upper pressure plate iii. The bottom pressureplate 50 rests against a flange 51 which is integral with the case. Acompact unit, as seen in Figure l, is thus provided when microphone isassembled and the ring so is screwed into position.

In considering the operation of this microphone after assembly, it isseen that the diaphragm or pressure sensitive element is mounted in amanner to respond to sound pressure impinging on oppostie surfacesthereof.

The pressure plates it? and at) are arranged with the slot openings 22,24, M and 46 disposed such that they open at spaced intervals on thecylindrical casing d. The four openings that are provided correspond,respectively, to the first, second, third and fourth elements referredto above. The openings are arranged in series relation. Two of theopenings 22 and as occupy extreme positions and the two openings 24 and4.4 occupy mean positions. As shown in Figure l, the openings arearranged alternatively on opposite sides of the plane of the diaphragm 6so that the successive openings in the series are connected toalternative acoustic chambers which include the annular grooves 20 and42.

in order that the second and third openings 24 and 44- provide twice thesensitivity to the diaphragms as the first and fourth openings theacoustical impedances of the secend and third openings are made half aslarge as the acoustical impedances than the first and fourth openings.

The diaphragm is mounted between the two pressure plates and is spacedtherefrom at the peripheral edge by the thin annular washers. Each ofthe slot Openings is connected to the acoustic chamber.

The acoustical circuit for the third order microphone, such asdescribed, is illustrated by a simplified electrical circuit shown inFigure 3, wherein:

R1=the lumped acoustical resistance of the slot opening 12 or pressurepoint;

R2=the lumped acoustical resistance of the second opening 24 or pressuresensitive point;

Ra the lumped acoustical resistance of the third opening 44 or pressuresensitive point;

R4=the lumped acoustical resistance of the fourth opening 46 or pressuresensitive point;

Rn =the lumped acoustical resistance of the diaphragm;

L1=the lumped acoustical inertance of the first opena;

L2=the lumped acoustical inertance of the second opena;

L3=the lumped acoustical inertance of the third opens;

L4=the lumped acoustical inertance of the fourth opens;

Ln=the lumped acoustical inertance of the diaphragm;

C1=the lumped acoustical compliance of the cavity on the left hand sideof the diaphragm as viewed in Figure 2;

Cz=the lumped acoustical compliance of the cavity on the other side ofthe diaphragm;

Cn=the lumped acoustical compliance of the diaphragm;

V1=the sound pressure at the first opening leading to one surface of thediaphragm;

V2=tl1e sound pressure at the third opening leading to the same surfaceof the diaphragm;

V =the sound pressure at the second opening leading to the other surfaceof the diaphragm;

V4=the sound pressure at the foruth opening leading to the same surfaceof the diaphragm as Vs;

in=tl1e acoustical volume current through the diaphragm.

If the sound pressure at all the apertures is identical at the sametime, the voltage output of the microphone will be 0. If the soundpressure p is incident upon the second aperture and the pressure at allthe other apertures is 0, the displacement of the diaphragm will betwice that obtaining when the same sound pressure is incident upon thefourth aperture and the pressure at all the other apertures is zero. Thesame analogy may be drawn with respect to apertures 1 and 3.

Referring particularly to Figure 3, in must be Zero when V1=V2=V3=V4.When V1=V and V2=Vs=V4=0, in must be half the value obtaining when V2=Vand where V is any arbitrary voltage. Similarly, when V4=V andV1=V2=Va=0, in must be half the value obtaining when V3=V andV1=V2=V4=0.

These conditions are satisfied by the present invention by designing themicrophone so that Ci cz, and

The voltage output will be zero when this condition is met and the soundpressure at all the apertures is identical at the same time.

Other impedance ratios and other hole spacings are possible and willsuggest themselves to those normally skilled in the art. For example ifthe separation between the second and third apertures is made equal tothe separations between the first and second and the third and fourthapertures, and if the impedances of the two diaphragm cavities areequal, the impedances of the first and fourth apertures must be threetimes the impedances of the second and third apertures.

This invention provides improved frequency response characteristicspossible by eliminating the long acoustic conduits which causeirregularity in the response frequency characteristics at the conduitresonant frequency. The end corrections for the impedance of the tubesas they lead into cavities are unnecessary in the present inventionwhere the length of these tubes is extremely short. It is also mucheasier to damp the microphone by the insertion of suitable acousticalresistance material and thus improve the overall response frequencycharacteristic. An additional advantage is that the symmetry of thediaphragm pressure plates is easy to obtain and proper balancing of themicrophone is dependent upon fewer precision parts.

The annular grooves within the pressure plates provide acousticalchambers which are relatively easily matched during the manufacture ofthe pressure plates. Proper design of the pressure plates Will assure aminimum amount of adjustment after manufacture.

It is seen that this microphone requires a minimum number of parts withonly the two pressure plates requiring any great degree of care in themanufacturing process. This, of course, permits easy assembly and lowercosts of construction.

The fluted diaphragm used in combination with the other parts of themicrophone aids in providing a microphone which responds accurately andreproduces truly and without rattle substantially all the frequencieswithin the normal audible range.

What is claimed is:

l. A third order, pressure gradient responsive microphone comprising apressure sensitive element, a pressure plate disposed on one side ofsaid pressure sensitive element, said pressure plate having an annulargroove to form an acoustic chamber, said pressure plate also having aplurality of spaced apart indentations connecting said annular groovewith the ambient, a second pressure plate disposed on the opposite sideof said pressure sensitive element, said second pressure plate having asecond annular groove to form a second acoustic chamber, and a pluralityof spaced apart indentations connecting said second annular groove tothe ambient.

2. A third order, pressure gradient responsive microphone comprising ahousing, a pressure sensitive element mounted within said housing, apressure plate disposed on one side of said pressure sensitive element,said pressure plate having an annular groove to form an acousticchamber, said pressure plate also having a plurality of spaced apartindentations connecting said annular groove with the ambient, one ofsaid indentations being so dimensioned as to provide a larger acousticalimpedance than one of the other indentations, a second pressure platedisposed on the opposite side of said pressure sensitive element, saidsecond pressure plate having a second annular groove to form a secondacoustic chamber, and a plurality of spaced apart indentationsconnecting said second annular groove to the ambient, one of saidindentations of said second pressure plate being so dimensioned as toprovide a lar er acoustical impedance than one of said otherindentations.

3. A third order, pressure gradient responsive micro phone comprising ahousing, a diaphragm having a fluted portion, a pressure plate disposedon one side of said diaphragm, said pressure plate having an annulargroove to form an acoustic chamber, said pressure plate also having aplurality of spaced apart indentations connecting said annular groovewith the ambient, said annular groove being in substantial alignmentwith said fluted portion of said diaphragm, a second pressure platedisposed on the opposite side of said diaphragm, said second pressureplate having a second annular groove to form a second acoustic chamberand a plurality of spaced apart indentations connecting said secondannular groove to the ambient, said second annular groove being insubstantial alignment with said fluted portion of said diaphragm.

4. A third order, pressure gradient responsive microphone comprising ahousing, a diaphragm having a fluted portion mounted within saidhousing, a pressure plate disposed on one side of said diaphragm, saidpressure plate having an annular groove to form an acoustic chamber,said pressure plate also having a plurality of spaced apart indentationsconnecting said annular groove with the ambient, one of saidindentations being so dimensioned as to provide a larger acousticalimpedance than one of the other indentations, said annular groove beingin substantial alignment with said fluted portion of said diaphragm, asecond pressure plate disposed on the opposite side of said diaphragm,said second pressure plate having a second annular groove to form asecond acoustic chamber, and a plurality of spaced apart indentationsconnecting said second annular groove to the ambient, one of saidindentations of said second pressure plate being so dimensioned as toprovide a larger acoustical impedance than one of said otherindentations, said second annular groove being in substantial alignmentwith said fluted portions of said diaphragm.

5. A third order, pressure gradient responsivemicrophone comprising apressure sensitive element, a pressure plate disposed on one side ofsaid pressure sensitive element, said pressure plate having an annulargroove to form an acoustic chamber, said pressure sensitive element alsohaving a pair of spaced apart indentations connecting said annulargroove with the ambient, one of said openings being so dimensioned as toprovide twice the acoustical impedance as the other, a second pressureplate disposed on the opposite side of said pressure sensitive element,said second pressure plate having a second annular groove to form asecond acoustic chamher, and a pair of spaced apart indentationsconnecting said second annular groove to the ambient, one of saidopenings of said second pressure plate being so dimensioned as toprovide twice the acoustical impedance as said other opening.

6. A microphone as defined in claim 5 wherein said indentations aredisposed in series in spaced apart relation alternately on oppositesides of said pressure sensitive element to provide a pair of extremeopenings and a pair of mean openings, said extreme openings each beingdimensioned to provide substantially twice the acoustic impedance ofother of said mean openings, successive ones of said series of openingsbeing connected to alternate sides of said acoustic chambers.

7. A third order, pressure gradient responsive microphone comprising apressure sensitive element, means connected with said pressure-sensitiveelement for converting vibrations thereof into corresponding electricalsignals, a pressure plate disposed on one side of said pressuresensitive element, said pressure plate having an annular groove to forman acoustic chamber, said pressure plate also having a pair of spacedapart indentations connecting said annular groove with the ambient, oneof said indentations being so dimensioned as to provide substantiallytwice the acoustic impedance as the other indentation, said pressureplate further having a central aperture, a spacing member adapted to fitinto said central aperture, means to attach said spacing member to saidpressure plate, adjustable means to vary the spacing between saidpressure plate and said pressure responsive element, a second pressureplate disposed on the opposite side of said pressure sentitive element,said second pressure plate having a second annular groove to form asecond acoustic chamber and a pair of spaced apart indentationsconnecting said second annular groove to the ambient, one of saidindentations of said second pressure plate being so dimensioned as toprovide substantially twice the acoustical impedance as said otherindentation.

8. A third order, pressure gradient responsive microphone comprising ahousing member, a pressure sensitive element disposed within saidhousing member, a pres- 7 sure plate disposed on one side of saidpressure sensitive element, said pressure plate having an annular grooveto form an acoustic chamber, said pressure plate also having a pluralityof spaced apart openings connecting said annular groove with theambient, a second pressure plate disposed on the opposite side of saidpressure sensitive element, said second pressure plate having a secondannular groove to form a second acoustic chamber and a plurality ofspaced apart openings connecting said second annular groove to theambient, said openings being disposed in series in spaced apart relationalternately on opposite sides of said pressure sensitive element toprovide a pair of extreme openings and a pair of mean openings, saidextreme openings each being dimensioned to provide substantially twicethe acoustical impedance as either of said mean openings, successiveones of said series of openings being connected to alternate sides ofsaid acoustic chambers.

9. A microphone as defined in claim 8 wherein said housing member isprovided with a series of apertures in alignment with said openings, anda retaining ring is provided to threadedly engage said housing member tohold said second pressure plate in an operative position.

References Cited in the file of this patent UNITED STATES PATENTS1,844,487 Tyrrell Feb. 9, 1932 2,475,782 Giannini July 12, 1949

